Fuel separation devices have been disclosed in, for example, U.S. Pat. Nos. 6,972,093 and 6,711,893. A fuel separation unit can accept a mixed feed stream, deliver the mixed feed stream to a permeable substrate, and separate the mixed feed stream into two streams, a retentate and a permeate. The feed stream can be a gas or a liquid. For example, If the separation unit is used to separate fuel such as gasoline, a mixed feed stream such as a fuel having a mixture of components can be heated and introduced to the fuel separation unit as a gas. The fuel can be separated into, for example, a relatively high Research Octane Number (RON) fuel portion and a relatively low RON fuel portion. The separated high RON fuel (the permeate) can then be cooled and delivered to a separate tank, to be used by the automobile when needed. An automobile may require high RON fuel during a high-output operation state such as at start up or upon acceleration. By providing a source of high RON fuel, the engine can perform better during high-output operation, For example, the ignition timing of the engine can be advanced, thereby increasing the output of the engine during these high-output operations.